This invention relates to a wearable display device, and more particularly to a wearable display using electrically switchable holographic optical elements.
There is a requirement for a compact see through data display capable of displaying image content ranging from symbols and alphanumeric arrays to high-resolution pixelated images. The display should be highly transparent and the displayed image content should be clearly visible when superimposed over a bright background scene. The display should provide full colour with an enhanced colour gamut for optimal data visibility and impact. A prime requirement is that the display should be as easy to wear, natural and non-distracting as possible with a form factor similar to that of ski goggles or, more desirably, sunglasses. The eye relief and pupil should be big enough to avoid image loss during head movement even for demanding military and sports activities. The image generator should be compact, solid state and have low power consumption.
The above goals are not achieved by current technology. Current wearable displays only manage to deliver see through, adequate pupils, eye relief and field of view and high brightness simultaneously at the expense of cumbersome form factors. In many cases weight is distributed in the worst possible place for a wearable display, in front of the eye. The most common approach to providing see through relies on reflective or diffractive visors illuminated off axis. Microdisplays, which provide high-resolution image generators in tiny flat panels, do not necessarily help with miniaturizing wearable displays because the requirement for very high magnifications inevitably results in large diameter optics. Several ultra low form factor designs offering spectacle-like form factors are currently available but usually require aggressive trade-offs against field of view, eye relief and exit pupil.
The optical design benefits of DOEs are well known including unique and efficient form factors and the ability to encode complex optical functions such as optical power and diffusion into thin layers. Bragg gratings (also commonly termed volume phase grating or holograms), which offer the highest diffraction efficiencies, have been widely used in devices such as Head Up Displays.
It is also known that diffractive optical elements can be used to provide virtual images for direct viewing or for viewing with the aid of optical systems. U.S. Pat. No. 6,052,540 by Koyama discloses a viewfinder device comprising a transmission hologram that can be located at a position other than in an image plane. The position of the virtual image formed by the transmission hologram is arranged to lie at the image plane of the optical system.
An important class of diffractive optical element known as an electrically Switchable Bragg Gratings (SBG) is based on recording Bragg gratings into a polymer dispersed liquid crystal (PDLC) mixture. Typically, SBG devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A Bragg grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the PDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting Bragg grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. In the absence of an applied electric field the SBG remains in its diffracting state. When an electric field is applied to the hologram via the electrodes, the natural orientation of the LC droplets is changed thus reducing the refractive index modulation of the fringes and causing the hologram diffraction efficiency to drop to very low levels. The diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from essentially zero to near 100%.
U.S. Pat. No. 5,942,157 by Sutherland et al. and U.S. Pat. No. 5,751,452 by Tanaka et al. describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. A recent publication by Butler et al. (“Diffractive properties of highly birefringent volume gratings: investigation”, Journal of the Optical Society of America B, Volume 19 No. 2, February 2002) describes analytical methods useful to design SBG devices and provides numerous references to prior publications describing the fabrication and application of SBG devices.
There is a requirement for a compact, lightweight wearable display providing a high brightness, high contrast information display with a large field of view, large exit pupil and a high degree of transparency to external light